A machine for applying a tread to a tire carcass, e.g. of the type described in Patent Application EP-1230235-A2, comprises a rotary drum supporting the carcass; a feed conveyor for feeding the tread to the rotary drum; a pressure roller contacting the tread between the drum and the feed conveyor; an actuating device for pushing the pressure roller against the tread with a force of adjustable intensity; and a control unit for regulating the intensity of the force produced by the actuating device as a function of an error variable calculated as the difference between the length of the remaining portion of the circumference of the carcass, and the length of the remaining portion of the tread. Other examples of machines for applying a tread to a tire carcass are described in Patents DE-2105765-A, U.S. Pat. No. 3,728,181-A1, U.S. Pat. No. 5,427,636-A1 and EP-0704296-A1.
The purpose of the pressure roller is to exert pulling force on, to permanently stretch, the tread; and a precise amount of permanent stretch must be produced on the tread to achieve the desired predetermined overlap of the two free ends of the tread when it is wound about the carcass.
When applying a cured new tread to a carcass as part of a tire retreading process involving no curing, the new tread must be cut taking into account the tread pattern, and so cannot be cut exactly to size with respect to the actual circumference of the carcass. The tread-stretching action of the pressure roller is therefore essential in ensuring correct application of the tread to the carcass. When applying a green tread to a carcass as part of the original tire manufacturing process or as part of a tire retreading and curing process, the new tread is cut exactly to size with respect to the actual circumference of the carcass, but may subsequently undergo a slight, unpredictable variation in length due to shrinkage caused by changes in temperature. In which case, the tread-stretching action of the pressure roller may prove useful in ensuring correct application of the tread to the carcass.
The control unit of known machines for applying a tread to a tire carcass, e.g. of the type marketed by SH under the trade name “Hitech Extru-Builder Baz 2160”, comprises a PID (Proportional-Derivative-Integral) control block, which regulates the intensity of the force produced by the actuating device as a function of the error variable.
Numerous tests show precise tread stretch by the pressure roller of known machines for applying a tread to a tire carcass to be extremely difficult to achieve. To increase tread stretch precision of the pressure roller, it has been proposed to increase gain of the integral contribution, which, however, results in oscillating and potentially unstable control, with obvious anomalous stress of the tread. To reduce oscillation, therefore, it has been proposed to increase gain of the derivative contribution, which in turn reduces control speed and prevents sufficient error reduction in the time taken to apply the tread.